Slush or mud pumps are commonly used for pumping drilling mud in connection with oil well drilling operations. Because of the need to pump the drilling mud through several thousand feet of drill pipe, such pumps typically operate at high pressures. Moreover, it is necessary for the mud to emerge from the drill bit downhole at a relatively high velocity in order to provide lubrication and cooling to the bit and to provide a vehicle for the removal of drill cuttings from the earth formation being drilled. Lastly, the pressure generated by the mud pump contributes to the total downhole pressure, which is used to prevent well blowouts.
The pistons and cylinders used for such mud pumps are susceptible to a high degree of wear during use because the drilling mud is relatively dense and has a high proportion of suspended abrasive solids. As the pump cylinder becomes worn, the small annular space between the piston and the cylinder wall increases substantially and sometimes irregularly. For these reasons, the seal design for such pumps is critical.
The high pressure abrasive environment in which the pumps must operate is especially deleterious to the seals since considerable friction forces are generated, and since the hydraulic pressures encountered during operation force the seal into the annular space between the cylinder wall and the piston. In some instances, the frictional forces may even detach the seal from the piston. In these instances, the edges of the seal can become damaged very quickly by the cutting or tearing action that occurs as a result of piston movement. Another problem with conventional mud pump seals is that they do not adequately "wipe" the cylinder wall, with the result that pressurized drilling mud seeps between the seal and the cylinder wall.
Attempts have been made to retain the seal in the piston so as to resist this frictional force. One conventional solution to this problem has been use of a metallic seal retainer which is disposed over the seal body and retained in place by snap rings. One disadvantage of this solution, however, is that the additional seal retaining element and its snap rings render the overall piston construction more expensive. A further disadvantage is that the seal is made somewhat less flexible and resilient than it would otherwise be, thus decreasing its ability to wipe the cylinder wall effectively.
Another conventional solution to the sealing problem comprises including a seal retaining ring or reinforcement in the seal itself. In this case, the retaining ring or reinforcement is molded into the seal material. As with the external retaining ring, this solution decreases the flexibility of the seal and increases its cost of manufacture.
It is common to incorporate the foregoing seals into piston heads wherein the seal is permanently affixed to the piston head. This is disadvantageous because the seal tends to wear much faster than the piston head, resulting in waste and unnecessary expense when the whole piston head has to be replaced because of wear to the seal member. It is therefore desirable to provide a piston seal that is removable from the piston head and thus can be replaced without requiring replacement of the whole piston head.
The nature of the mud pump operating environment makes it difficult to effectively address these issues. It is, therefore, desired to develop a new and improved replaceable seal for a reciprocating mud pump piston that overcomes the foregoing difficulties while providing better wear properties and more advantageous overall results.